It is a commonplace requirement in the production and use of compressed air that the air coming directly from the compressor is unsuitable for use. It is therefore equally commonplace for preparation techniques to be applied to the compressed gas before it is used. For example, the pressure of the gas may be varied using a regulator, a lubricating oil may be added and the air may be cleaned and dried after compression and before it is used. An example of drying techniques employed include pressure swing adsorption (PSA) where the compressed air is passed through a drying media contained in a drying vessel under high pressure. The drying media is then regenerated by decreasing the pressure in the drying vessel and a small quantity of the dry air is passed back through the media at reduced pressure thereby removing moisture resulting in regeneration.
Prior to drying it is also commonplace for the compressed air to be passed through a filter to remove aerosol particles, such as micro droplets of oil, and excess water in droplet form. One type of these filters, which is commonly used, has three major components: an end cap; a filter element (or cartridge); and a filter bowl. The end cap and filter bowl form the pressure containing external components of the filter housing and since they receive compressed air under pressure are operating with a large pressure differential between their inside and outside surfaces. Typically compressed air at up to 20 bar g (above the external atmospheric pressure) is experienced between the internal and external surfaces of the end cap, although more typically in the range 6 to 10 bar g. The end cap is a complex component generally formed by casting whereas the filter bowl has a tubular body sealed with a base lid. For smaller filter bowls (up to around 10 cm diameter) the bowl is generally formed by casting. However, for larger bowl sizes, the tubular body of the bowl is most commonly formed by extrusion. The extruded Aluminium is significantly less porous than a cast product which often requires impregnation to seal it to make it useable with compressed air. Furthermore, when testing for certification marks (e.g. a Canadian Registration Number) in some jurisdictions, the testing applied to extruded products is different or much less stringent that those applied to cast products. The approval process for casting is significantly more complex due to the increased likelihood of imperfections (typically small pores formed by gas bubbles dissolved in the molten metal) in the internal structure of the casting.
Preferred embodiments of the present invention seek to overcome the above described problems with the prior art.